Handbook for Sound Engineers

372 Chapter 13

device and have good thermal conductivity across its
entire area. The specific thermal resistance U of inter-
face materials used for heatsinks and insulating devices
is shown in Table 13-3.
The thermal resistance T for these materials can be
determined by the equation

(13-5)

where,
U is the specific thermal resistance,
t is the material thickness in inches,
A is the area in square inches.

For instance, a square copper plate 4 inches per side
and inch thick would have a T of 0.00078°C/W,

while a mica insulator 0.003 inch thick with a diameter

of 1 inch would have a T of 0.25°C/W. If the semicon-

ductor dissipates 100 W, the temperature drop across the

copper plate would be 0.07°C (0.13°F) and across the

mica washer it would be 25°C (45°F). In transistor

replacement in older equipment, it would be best to

replace the mica insulator with a new type of insulator.

In the selection of a heatsink material, the thermal

conductivity of the material must be considered. This

determines the thickness required to eliminate thermal

gradients and the resultant reduction in emissivity. An

aluminum fin must be twice as thick as a comparable

copper fin, and steel must be eight times as thick as

copper.

Except for the smallest low-current solid-state

devices, most devices must use a heatsink, either built

in or external.

Space for heatsinks is generally limited, so the

minimum surface area permissible may be approxi-

mately calculated for a flat aluminum heat plate by

(13-6)

where,

W is the power dissipated by the device,

'T is the temperature differences between the ambient

and case temperature in qC.

The approximate wattage dissipated by the device

can be calculated from the load current and the voltage

drop across it

(13-7)

where,

IL is the load current,

VD is the voltage drop across the device.

Figure 13-6. Transistor/heatsink mounting kits. Courtesy
Delco Electronics Corp.

Transistor

MICA insulator

11 / 8 in^2 × 0.001 in to

0.002 in thick

Insulating bushings

use one for mounting on

material^1 / 8 in to^15 / 64 in thick.

Use two for material of

½ in or greater thickness

Copper or aluminum heat

sink or chassis

MICA insulator

Metal washer

Solder lug

#10-32 hex. nut

#4-40 ×½ R.H. screw (2)

Transistor

MICA insulator

Copper or aluminum heat sink

or chassis

Insulating bushing (2)

Lockwasher

Solder lug

#4-40 nut (2)

T Ut

A

-----=

(^1) » 8
Table 13-3. Specific Thermal Resistance p of
Interface Materials, °C in/W
Material U
Still air 1200
Mylar film 236
Silicone grease 204
Mica 66
Wakefield Type 120 Compound 56
Wakefield Delta Bond 152 47
Anodize 5.6
Aluminum 0.19
Copper 0.10
Courtesy of Wakefield Engineering, Inc.
A 133
W
'T
= ------ - in^2
WI= LVD